The Inner Ear
The inner ear is the most distal part of the ear, housing the vestibulocochlear organs. It has two main functions:
- To convert mechanical signals from the middle ear into electrical signals, which can transfer information to the auditory pathway in the brain.
- To maintain balance by detecting position and motion.
In this article, we shall look at the anatomy of the inner ear – its position, structure and neurovascular supply.
Anatomical Position and Structure
The inner ear is located within the petrous part of the temporal bone. It lies between the middle ear and the internal acoustic meatus, which lie laterally and medially respectively. The inner ear has two main components – the bony labyrinth and membranous labyrinth.
- Bony labyrinth – consists of a series of bony cavities within the petrous part of the temporal bone. It is composed of the cochlea, vestibule and three semicircular canals. All these structures are lined internally with periosteum and contain a fluid called perilymph.
- Membranous labyrinth – lies within the bony labyrinth. It consists of the cochlear duct, semicircular ducts, utricle and the saccule. The membranous labyrinth is filled with fluid called endolymph.
The inner ear has two openings into the middle ear, both covered by membranes. The oval window lies between the middle ear and the vestibule, whilst the round window separates the middle ear from the scala tympani (part of the cochlear duct).
The bony labyrinth is a series of bony cavities within the petrous part of the temporal bone. It consists of three parts – the cochlea, vestibule and the three semicircular canals.
The vestibule is the central part of the bony labyrinth. It is separated from the middle ear by the oval window, and communicates anteriorly with the cochlea and posterioly with the semicircular canals. Two parts of the membranous labyrinth; the saccule and utricle, are located within the vestibule.
The cochlea houses the cochlea duct of the membranous labyrinth – the auditory part of the inner ear. It twists upon itself around a central portion of bone called the modiolus, producing a cone shape which points in an anterolateral direction. Branches from the cochlear portion of the vestibulocochlear (VIII) nerve are found at the base of the modiolus.
Extending outwards from the modiolus is a ledge of bone known as spiral lamina, which attaches to the cochlear duct, holding it in position. The presence of the cochlear duct creates two perilymph-filled chambers above and below:
- Scala vestibuli: Located superiorly to the cochlear duct. As its name suggests, it is continuous with the vestibule.
- Scala tympani: Located inferiorly to the cochlear duct. It terminates at the round window.
There are three semicircular canals; anterior, lateral and posterior. They contain the semicircular ducts, which are responsible for balance (along with the utricle and saccule).
The canals are situated superoposterior to the vestibule, at right angles to each other. They have a swelling at one end, known as the ampulla.
The membranous labyrinth is a continuous system of ducts filled with endolymph. It lies within the bony labyrinth, surrounded by perilymph. It is composed of the cochlear duct, three semicircular ducts, saccule and the utricle.
The cochlear duct is situated within the cochlea and is the organ of hearing. The semicircular ducts, saccule and utricle are the organs of balance (also known as the vestibular apparatus).
The cochlear duct is located within the bony scaffolding of the cochlea. It is held in place by the spiral lamina. The presence of the duct creates two canals above and below it – the scala vestibuli and scala tympani respectively. The cochlear duct can be described as having a triangular shape:
- Lateral wall – Formed by thickened periosteum, known as the spiral ligament.
- Roof – Formed by a membrane which separates the cochlear duct from the scala vestibuli, known as the Reissner’s membrane.
- Floor – Formed by a membrane which separates the cochlear duct from the scala tympani, known as the basilar membrane.
The basilar membrane houses the epithelial cells of hearing – the Organ of Corti. A more detailed description of the Organ of Corti is beyond the scope of this article.
Saccule and Utricle
The saccule and utricle are two membranous sacs located in the vestibule. The utricle is the larger of the two, receiving the three semicircular ducts. The saccule is globular in shape and receives the cochlear duct.
Endolymph drains from the saccule and utricle into the endolymphatic duct. The duct travels through the vestibular aqueduct to the posterior aspect of the petrous part of the temporal bone. Here, the duct expands to a sac where endolymph can be secreted and absorbed.
The semicircular ducts are located within the semicircular canals, and share their orientation. Upon movement of the head, the flow of endolymph within the ducts changes speed and/or direction. Sensory receptors in the ampullae of the semicircular canals detect this change, and send signals to the brain, allowing for the processing of balance.
The bony labyrinth and membranous labyrinth have different arterial supplies. The bony labyrinth receives three arteries, which also supply the surrounding temporal bone:
- Anterior tympanic branch (from maxillary artery).
- Petrosal branch (from middle meningeal artery).
- Stylomastoid branch (from posterior auricular artery).
The membranous labyrinth is supplied by the labyrinthine artery, a branch of the inferior cerebellar artery (or, occasionally, the basilar artery). It divides into three branches:
- Cochlear branch – supplies the cochlear duct.
- Vestibular branches (x2) – supply the vestibular apparatus.
Venous drainage of the inner ear is through the labyrinthine vein, which empties into the sigmoid sinus or inferior petrosal sinus.
The inner ear is innervated by the vestibulocochlear nerve (CN VIII). It enters the inner ear via the internal acoustic meatus, where it divides into the vestibular nerve (responsible for balance) and the cochlear nerve (responsible for hearing):
- Vestibular nerve – enlarges to form the vestibular ganglion, which then splits into superior and inferior parts to supply the utricle, saccule and three semicircular duct.
- Cochlear nerve – enters at the base of the modiolus and its branches pass through the lamina to supply the receptors of the Organ of Corti.
The facial nerve, CN VII, also passes through the inner ear, but does not innervate any of the structures present.
Clinical Relevance: Meniere’s Disease
Meniere’s disease is a disorder of the inner ear, characterised by episodes of vertigo, low-pitched tinnitus and hearing loss.
The symptoms are thought to be caused by an excess accumulation of endolymph within the membranous labyrinth, causing progressive distension of the ducts. The resulting pressure fluctuations damage the thin membranes of the ear that detect balance and sound.